1. Field
This application related to gas tungsten arc welding and more particularly to a titanate flux that substantially increases weld penetration.
2. Background
The gas tungsten arc welding (GTAW) process is an arc welding process that uses an arc between a non-consumable tungsten electrode and the work piece to cause localized coalescence of the base material. The GTAW process is used to produce high quality welds in a variety of materials. Applications include welding of sheet, plate, tube, and castings for use in aerospace, power generation, shipbuilding, and other industries. GTAW can be used with filler metals or consumable inserts to produce welds in thick sections. Autogenous welds (welds without filler metals) can be made in thin sections or for root passes in thick sections.
The primary limitation of the process is low productivity due to low deposition rates and shallow penetration. The inability of GTAW to produce welds with deep penetration limits the thickness of material that can be reliably joined to less than approximately 0.10 in (2.5 mm) thick. Materials greater than 0.1 in (2.5 mm) thick typically require weld joint preparation and multiple passes to fill the weld joint. GTAW welds are also affected by heat-to-heat compositional variations in the material being welded. While variable penetration is most often encountered in stainless steel and nickel-based alloys, this phenomenon occurs in other materials as well.
A method of increasing both the amount and consistency of penetration in gas tungsten arc (GTA) weldments is to apply a thin layer of flux to the surface of the part or joint to be welded prior to welding. The use of traditional fluxes for the GTAW process is not required since shielding and arc stabilization are provided by the use of an external shielding gas. Fluxes used for GTAW to improve penetration are inherently different than those used for soldering, brazing, or other arc welding processes since they neither clean the surface of the part nor protect the weld pool from oxidation.
The use of flux for increasing penetration in mild steel materials has been described by several authors in the former Soviet Union. An article by E. D. Raimond et al. titled xe2x80x9cWelding of High Strength Steel Using Activating Fluxes in Powder Formxe2x80x9d Svar. Proiz, No. 6, pp. 18-19, suggests that the use of Soviet Flux FS-71 increased GTAW penetration in steel by 50-100 percent. No compositional details were given for the flux. A later article by O.E. Ostroviski entitled xe2x80x9cThe Effect of Activating Fluxes on the Penetration Capability of the Welding Arc and the Energy Concentration in the Anode Spotxe2x80x9d Svar. Proiz, No. 3, pp. 3-4, 1977, reveals that the composition of flux FS-71 is 57.3 percent SiO2, 6.4 NaF, 13.6 TiO2, 13.6 Ti, and 9.1 Cr2O3. Another paper by L. E. Eroshenko et al, titled xe2x80x9cAn Examiantion of the Glow of Anode Vapors for the Evaluation of the Technological Characteristics of the Arc Running in Argon, Avt. Svarka, 1979, No. 9, pp. 33-35, evaluated the enhanced penetration caused by fluorides of several alkali and alkaline-earth elements. The effects of individual fluorides were studied and the fluorides evaluated in the study were used as a basis for selecting components for titanium and steel GTAW fluxes. A similar approach to the Soviet flux design is described in U.S. Pat. No. 5,525,163 by H. R. Conaway et. al. They claim that the use of 7 to 59 percent LIF promotes penetration in 321 austenitic stainless steel. They infer that this ingredient will promote enhanced penetration in other materials such as carbon steel as well. Paskell describes a flux composed of TiO or TiO2 (50%), Cr2O3 (40%) and SiO2 (10%) in U.S. Pat. No. 5,804,792 that is used to increase the penetration in stainless steel.
The flux reported by Ostrovski (FS-71) and flux no. 69 reported by Conaway (23.6 Al2C3, 39.4 LiF, 15.7 MgO, 5 B2O3, and 15.7 Fe2O3) were evaluated the instant effort on SA178 Gr. C mild steel pipe. Neither flux spread well or produced consistently improved penetration above what was measured without flux. Additionally, these fluxes both contain fluorides which can increase the risk of corrosion in some environments and which generated a considerable amount of fluoride based fume which is can be hazardous as well.
It is an object of the present invention to provide a flux composition for use with the GTA process that improves weld penetration in a variety of steels including stainless steel and also in nickel-based alloys.
It is an object of the present invention to reduce the cost of the GTA process by reducing the time and effort in joint preparation.
It is an object of the present invention to reduce the number of passes in a multi-pass gas tungsten arc weld.
It is an object of the present invention to reduce the distortion in a gas tungsten arc weld.
It is an object of the present invention to produce welds with mechanical properties that are not degraded through the use of the flux composition.
It is an object of the present invention to produce welds with a weld quality that is not degraded through the use of the flux composition.
The foregoing and other objects, features and advantages of the invention will become apparent from the following disclosure in which one or more preferred embodiments of the invention are described in detail. It is contemplated that variations in procedures may appear to a person skilled in the art without departing from the scope of or sacrificing any of the advantages of the invention.
The above objects are met in the present invention by using a xe2x80x9ctitanatexe2x80x9d penetration flux to achieve deep penetration welds in gas tungsten arc welding (GTAW). As used here, a xe2x80x9ctitanatexe2x80x9d is a salt and/or complex oxide of titanium such as TiO3xe2x88x922, TiO4xe2x88x924, Ti2O5xe2x88x922,Ti2O7xe2x88x924, and Ti3O72. The titantate is selected from alkaline-earth titanates, transition metal titanates, and alkali metal titanates such as Na2Ti3O7 or K2TiO3. Only a small amount of the titanate alone applied to the weld zone as a thin paste by mixing with a carrier fluid such as an alcohol or ketone or as part of a wire filler is sufficient to promote deep penetration GTAW of a wide variety of metals including carbon, chrome-molybdenum, and stainless steels as well as nickel-based alloys.
To control arc wander, bead consistency, and slag and surface appearance of the gas tungsten arc weldments, a wide variety of additional components may be optionally added to the titanate flux including transition metal oxides such as TiO, TiO2, Cr2O3, and Fe2O3, alkali oxides, alkaline-earth oxides, silicon dioxide, manganese silicides, alkali fluorides, alkaline-earth fluorides, alkali chlorides, and alkaline-earth chlorides. However, the toxicity of the plumes from halides and resulting weld corrosion may preclude halide use.
During the development of the titanate flux, it was found that a composition of TiO and/or TiO2, Cr2O3, and Fe2O3 gave good weld penetation in GTAW but with heat-to-heat variation in carbon steel and nickel-based alloys. Such a flux has about 5-40 wt % Fe2O3, 20-70 wt % Cr2O3, and 15-70 wt % of TiO or TiO2 or both with a more preferred composition having about 6 wt % Fe2O3, 60 wt % Cr2O3, 22 wt % TiO2 and 12 wt % TiO.
The method of using the titanate flux is quite simple. The flux is applied to the weld zone of the metal components to be joined as a thin paste, as part of a filler wire or even as part of the shielding gas. The metal components are welded together using a gas tungsten arc welding torch.
The foregoing and other objects, features and advantages of the invention will become apparent from the following disclosure in which one or more preferred embodiments of the invention are described in detail and illustrated in the accompanying examples. It is contemplated that variations in compositions and their use in the welding process may appear to a person skilled in the art without departing from the scope of or sacrificing any of the advantages of the invention.
In its basic form the present invention features the use of a xe2x80x9ctitanatexe2x80x9d to enhance weld penetration in the gas tungsten arc welding process. As used here, a xe2x80x9ctitanatexe2x80x9d is a salt and/or complex oxide of titanium such as TiO3xe2x88x922, TiO4xe2x88x924, Ti2O5xe2x88x922, Ti2O7xe2x88x924 , and Ti3O7xe2x88x922 Illustrative titanate include alkali metal titanates (M2TiO3, M2Ti2O5, M2Ti3O7) such as Na2Ti3O7, K2TiO3, K2Ti2O5; alkali earth metal titanates (M2TiO4) such as MgTiO3, SrTiO3, BaTiO3, and CaTiO3, and transition metal titanates (M2TiO4, MTiO3) such as FeTiO3 and ZnTiO4. In addition to improving weld penetration, that is, weld depth, these compositions are also capable of reducing heat-to-heat penetration variations.
The titanate compounds typically are used in the form of high-purity powders of about xe2x88x92325 mesh or finer. These penetration enhancing titanate compounds can be applied to the weld zone of the metal components to be welded by mixing the powders with a suitable carrier fluid such as methanol, ethanol, methyl ethyl ketone or other suitable carrier fluid to form a paste and applied to the component surface. Alternatively the titanate compound can also be included as part of a filler wire or rod and introduced into the weld pool through wire additions.
The primary purpose of the titanate compound is to improve penetration and reduce heat-to-heat variations in penetration. When compared to autogenous welds produced without flux, the penetration enhancing titanate compounds produce welds of superior penetration and at least comparable weld quality. Further these titanate compounds reduce the cost of welding through simplification of joint preparation, reduced consumption of filler wire, and reduced weld distortion. Titanate salts, by themselves, have been shown to enhance weld penetration with a wide variety of metals and alloy compositions including carbon steel, chrome-molybdenum steels, low alloy steel, austenitic stainless steel, and nickel-based alloys. Over 320 flux compositions were evaluated with some of the better compositions shown in Table 1.
These fluxes consist of at least one titanate compound such as Na2Ti3O7 or K2TiO3 Optionally these titanate penetrations fluxes may further comprise oxides of transition metals, oxides of alkali and alkaline-earth elements, silicon dioxide, and manganese silicide to control arc wander and improve bead consistency, slag appearance, and flux spreading. Typically transition metal oxides such as Cr2O3, TiO, TiO2, and Fe2O3 can be added to the titanate in amounts up to about 75 wt %. Cr2O3 is preferably used in the range of about 20-60 wt % with about 35-45 wt % giving consistent results when used with a variety of steels and nickel-based alloys. Fe2O3 is used in a range of about 5-40 wt % with about 10-25 wt % preferred and 15-20 wt% giving consistent results over the range of metals used. TiO or TiO2 or both are used with the titanate in amounts of about 15-70 wt % with 20-60 wt % preferred and 45-55 wt % giving consistent results for the various metals used. Silicon dioxide is used in amounts of about 3-30 wt % with 5-25 wt % preferred and about 6-15% giving good results for all metals tested. Manganese silicides are used in amounts of about 3-15 wt % with 5-10 wt % being preferred. The addition of halides to these titanate compositions does not degrade weld performance. Fluorides and chlorides are used in amounts of about 5-60 wt % with 20-45 wt % preferred and 35-40 wt % give goods results for most materials. Depending on alloy type, the titanate composition of these compounds can be adjusted to optimize weld characteristics for a given alloy system.
It is to be realized that the titanate compound is the controlling compound in these formulations which gives rise to deep penetration welds. Only a small amount of titanate need be applied to the weld pool to achieve deep penetrations welds. The weld penetration achieved with a titanate does not appear to depend in an interactive way on the amounts of other materials added to the flux. That is, the depth of weld penetration depends on the presence of titanate alone rather than on other materials and the interaction between these materials and the titanate. The wt % of titanate in a composition with other components can be minimal with regard to the other components of the flux, the filler material, inert materials, and/or the fluid carrier and yet still preserve its ability to afford good weld penetration. Generally the requisite amount of titanate in a particular composition should be sufficient to afford a thin open or closed coating of a xe2x88x92325 mesh titanate when all other components are removed.